1. Field of the Invention
The present invention relates to motion sensors and, more particularly, to a motion sensor having a movable member that requires no electrical connections.
2. Description of the Prior Art
Motion sensors are commonly used in high precision systems to monitor the linear or rotary movement of an object with respect to another object. For example, position and velocity sensors are commonly used by control systems that control the operation of robots and aircraft tracking systems. Also, position sensors are commonly used with gyroscopes that are employed by aircraft guidance systems.
Several concerns must be addressed when choosing a motion sensor for use in a high precision system. The information produced by the motion sensor should be in a form that is useful to those components of the system that receive it. Systems currently employed commonly use a general purpose computer or a microprocessor to coordinate the flow of information and to process the information. Therefore, it is highly desirable that the motion sensor produce information in digital form. Further, while most physical systems are designed or programmed to operate in a Cartesian coordinate system, many systems, such as robotic systems, operate and sense motion in polar, or other non-Cartesian, coordinate systems. The information produced by the system must be translated to Cartesian coordinates before it can be made useful. That is, the sensor of the system commonly produces an angle output, and the transcendental functions of the angle must be generated to resolve the vector representing the angle into its X-, Y- and Z- components.
Of further concern is the fact that a motion sensor that is employed by certain systems, such as gyroscope systems, should generate as little torque as is possible, since the torque generated by the sensor tends to degrade the performance of the system. A motion sensor can generate reactance torque in two ways. Both types of reactance torque are generated by a device that is commonly called an inductosyn, or a variable transformer sensor. An inductosyn is a mutually coupled magnetic device that employs a stationary member, or stator, and a movable member, or rotor, that is secured to and moved by the object whose motion is to be determined. Each member includes a conductor formed into a number of coils. The surfaces of the members that contain the coils confront each other and one set of coils is energized, thereby inducing an electrical current in the remaining set of coils. As the rotor moves, each coil of the energized conductor moves through positions in which it is located midway between adjacent coils of the nonenergized conductor, and in which the current induced in the nonenergized coils is a minimum, and in which each coil directly confronts a coil of the nonenergized conductor, in which the current provides an indication of the position of the rotor. However, when the coils of each conductor are not completely aligned, there exists a torque on the members that acts in a direction that tends to align the two sets of coils. Frictional torque is also generated by the apparatus of the inductosyn that is required to provide electrical communication between the system and the electrical conductors on the rotor and stator. Therefore, aside from its unsuitablility due to size and weight considerations, the inductosyn is unsuitable for gyroscope applications due to the level of torque it generates. Although inductosyns formed using thin film deposition techniques are smaller and lighter than conventional inductosyns, they still generate an unacceptably high level of torque for many applications requiring a high level of precision. Also, the contacting of the movable member by the current conductor, which is electrically connected to the stationary member of the device, generates undesirable electrical noise in the signal path.
The following United States Patents Nos. disclose motion sensors that either require electrical contact between a moving member and the system that receives information from the sensor, or that have two sets of coils that generate electromagnetic reactive torque as the sensor is operating:
______________________________________ 2,650,352 2,867,783 3,148,347 3,441,888 2,671,892 2,900,612 3,202,948 3,596,222 2,685,070 2,915,721 3,281,746 3,758,845 2,799,835 2,921,280 3,332,144 3,772,587 2,844,802 2,964,721 3,431,525 4,463,333 ______________________________________
U.S. Pat. No. 3,611,813 shows a tachometer consisting of two circular members, each of which includes a conductor forming an arcuately distorted generally periodic pattern. The conductor on the stator forms two balanced legs of a bridge which produce a zero output signal in the absence of the rotor. When the rotor is rotating proximate the stator, the impedance of the conductors on the stator changes and the rotor modulates the signal produced by the stator. The frequency of modulation increases with the speed of the rotor.
Accordingly, there exists the need for a motion sensor that reduces the amount of electromagnetic and frictional reactance torque that is produced by the sensor, that produces a signal in digital form, and that is particularly well-adapted to include circuitry for providing a signal that is relatively noise free, and that represents a function of the angular position of the rotor with respect to the stator.